15.7: The Metaclass of Metaclass Is an Instance of Metaclass

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The final question to be answered is: what is the class of Metaclass class? The answer is simple: it is a metaclass, so it must be an instance of Metaclass, just like all the other metaclasses in the system (see Figure \(\PageIndex{1}\)).

All metaclasses are instances of the class Metaclass. — Figure \(\PageIndex{1}\): All metaclasses are instances of the class Metaclass, even the metaclass of Metaclass.

Figure \(\PageIndex{1}\) shows how all metaclasses are instances of Metaclass, including the metaclass of Metaclass itself. If you compare Figures 15.6.1 and \(\PageIndex{1}\) you will see how the metaclass hierarchy perfectly mirrors the class hierarchy, all the way up to Object class.

The following examples show us how we can query the class hierarchy to demonstrate that Figure \(\PageIndex{1}\) is correct. (Actually, you will see that we told a white lie — Object class superclass \(\Rightarrow\) ProtoObject class, not Class. In Pharo, we must go one superclass higher to reach Class.)

Collection superclass
>>> Object

Collection class superclass
>>> Object class
Object class superclass superclass
>>> Class "NB: skip ProtoObject class"
Class superclass
>>> ClassDescription
ClassDescription superclass
>>> Behavior
Behavior superclass
>>> Object

Collection class class
>>> Metaclass
Object class class
>>> Metaclass
Behavior class class
>>> Metaclass

Metaclass class class
>>> Metaclass
Metaclass superclass
>>> ClassDescription